(1) Field of the Invention
This invention relates to a process for producing a film or a film-like product of a non-solvent type thermosetting resin, which is substantially free from a thermoplastic resin.
The film or film-like product obtained by the process of the present invention exhibits excellent uniformity in thickness and viscosity, and the thermosetting resin constituting the film or film-like product is homogeneous and free from a cross-linked infusible product of the thermosetting resin. Such a film or film-like product is useful as an adhesive film, particularly as a matrix resin of a film form used for a fiber-reinforced plastic prepreg.
By the term "film or film-like product" used herein we mean a film, sheet or another analogous thin product. This term is hereinafter referred to as "film" for brevity.
(2) Description of the Prior Art
In general it is difficult to produce a thermosetting resin film. Most conventional thermosetting resin films have been prepared by a process wherein a solution of a thermosetting resin in a suitable solvent is coated onto a release sheet and, then, the coated layer is heated tp volatilize the solvent. Thermosetting resins are readily converted into a three-dimensional cross-linked structure upon heating. Thus, when a solvent-free thermosetting resin film is produced by the above-mentioned conventional process, the resulting film is liable to possess infusible portions due to inevitable non-uniformity in heating and, consequently, not be uniform in its thermosetting properties. Furthermore, it is essential to remove the solvent from the coated thermosetting resin solution in the course of producing the solvent-free thermosetting resin film. The removal of the solvent necessitates the employment of a complicated production apparatus which requires a large amount of energy.
In order to obviate the above-mentioned defects, some proposals have heretofore been made for the production of a non-solvent type thermosetting resin film. For example, U.S. Pat. No. 3,784,433 discloses a process wherein a thermosetting resin, such as an epoxide resin, which is heated to about 260.degree. C. and in which a thermoplastic resin, such as a polysulfone, has been dissolved, is cooled to about 120.degree. C.; curing agent is added to the cooled resin mixture; and then, the resin mixture is pressed or cast into a film. However, this process has the following disadvantages. First, a salient amount, e.g., about 20% by weight, of a thermoplastic resin, such as a polysulfone, must be incorporated in a thermosetting resin, such as an epoxide resin, in order to obtain the intended non-solvent type thermosetting resin film. Secondly, the thermosetting resin must be exposed to an undesirably high temperature, e.g., about 260.degree. C., in order to dissolve the polysulfone therein. Thirdly, it is very difficult to continuously produce a film of uniform thickness by the pressing or casting procedure.